This application contains subject matter that is related to the subject matter of the following application, which is assigned to the same assignee as this application and filed on the same day as this application. The below listed application is hereby incorporated herein by reference in its entirely:
xe2x80x9cLight-Emitting Modulexe2x80x9d by Yabe et. al.
xe2x80x9cOptical Modulexe2x80x9d by Takagi et. al.
1. Field of Invention
This invention relates to an optical module, especially relates to an optical signal source used in a WDM (Wavelength Division Multiplexing) communication.
2. Related Prior Art
In the WDM communication, the wavelength interval to the adjacent channel is defined to be 0.8 nm. This regulation means that the absolute accuracy superior than xc2x10.1 nm is required for the signal wavelength of respective channel. A semiconductor laser, such as DFB laser (Distributed Feedback Laser) and DBR (Distributed Bragg Reflector), is utilized for the signal source of the WDM system.
These feedback lasers have a sharp oscillation spectrum with a typical bandwidth less than 50 GHz. However, since the Bragg grating formed within a laser chip solely determines the oscillation wavelength, it would be quite difficult to get the desired wavelength due to the uncertainty of the manufacturing process parameter.
It is also known that the oscillation wavelength of individual lasers can be adjusted by the feedback control after the completion of the production. The method is: 1) dividing the output light from the optical module, 2) monitoring the divided light with a spectrum analyzer, and 3) adjusting the temperature of the laser and the injection current to the laser, thus controlling the oscillation wavelength. However, this technique uses the optical spectrum analyzer and is quite impossible to apply to the WDM system, which requires a plurality set of such large-scale equipment for respective optical signal source.
Another example is disclosed in U.S. Pat. No. 5,825,792, in which a parallel plates etalon is used for the controlling of the oscillation wavelength. In the ""792 patent, two optical detectors monitor a divergent light emitted from the back facet of the laser through the etalon placed with an angle for the light. By feed backing the differential signal of two detectors to a temperature of the laser, the oscillation wavelength is effectively adjusted. This method realizes the precisely controlled oscillation wavelength, but requires a precise adjustment of the rotational angle of the etalon to the divergent light beam of the laser.
The object of the present invention is to provide a light-emitting module that enables to control both of the oscillation wavelength and the optical output power with high accuracy within a compact sized housing.
A light-emitting module according to the present invention comprises a second optical detector for monitoring a light from the semiconductor laser not through an etalon and a first optical detector for monitoring light from the laser through the etalon. The light through the etalon reflects optical properties both of the laser and the etalon, while the light not through the etalon merely shows the properties of the laser. The optical property of the etalon depends on a thickness and shows the transmittance with a periodicity.
Another aspect of the invention is that the etalon has a second portion on which an anti-reflection film is coated and a second portion. Light transmitted through the second portion does not show a periodic behavior based on the thickness of the etalon and merely reflects the characteristic of the laser. On the other hand, light through the first portion on which any anti-reflection film is provided has periodic characteristics reflecting the etalon and the laser.
The fluctuation of the oscillation wavelength of the laser appears as a phase shift of the periodic characteristic of light transmitted through the etalon. Therefore, by monitoring light through the etalon, the just present oscillation wavelength is detected and by monitoring light not through the etalon, the present power of the laser is obtained.
In the invention, it is preferable to split light from the laser before the etalon and to detect split light for monitor light not through the etalon. The light splitting device can locate either in the front side of the laser or the backside of the laser.
It is further preferable to place a lens between the laser and the etalon device for converting divergent light from the laser into a collimated light. Moreover, by using a wedge shape etalon, the oscillating wavelength of the laser can be selected merely sliding the etalon along a direction normal to the optical axis.
The present invention provides a thermoelectric cooler for adjusting temperature of the laser. The temperature is controlled by the signal from the detector that monitors light through the etalon, thus defines the oscillation wavelength.
The invention may also provide an adjusting circuit of the output optical power of the laser.
The signal from the detector that monitors light not through the etalon can maintains the output power of the laser.